Imaging spectroscopy is widely used in remote sensing applications. One type of interferometric spectrometer used to supply spectral data for many remote sensing applications is called a Fourier Transform Spectrometer (FTS). A common form of an FTS employs a Michelson interferometer with one arm having a variable optical path length. The variable optical path length may be implemented using a movable mirror. By scanning the movable mirror over some distance, an interference pattern or interferogram is produced that encodes the spectrum of the source. The FTS uses the Discrete Fourier Transform (DFT) or its faster algorithm, the Fast Fourier Transform (FFT), to convert the auto-correlation (each spectral amplitude encoded as the amplitude of a cosine signal) to physical spectra. The encoded spectrum is the Fourier transform of the source.
Referring to FIG. 1A, there is illustrated a block diagram of one example of an optical configuration of a conventional FTS using a scanning Michelson interferometer implemented with a movable mirror. In this example, the FTS includes two mirrors 105, 110 with a beamsplitter 115 positioned between them. Mirror 105 is a fixed mirror and mirror 110 is a movable mirror. Electromagnetic radiation 120 incident on the beamsplitter 115 from a radiation source (not shown) is divided into two parts, each of which propagates down one of the two arms and is reflected off one of the mirrors. Radiation 120a in a first optical path is reflected by the beamsplitter 115 and reflected by the fixed mirror 105. On the return, the radiation 120a is again split by the beamsplitter 115, such that 50% of the radiation is reflected back to the input, and the remainder is transmitter through the beamsplitter to a focal plane array 125. Radiation 120b in a second optical path is transmitted through the beamsplitter 115, and reflected by the movable mirror 110 which imparts a modulation to the radiation (motion of the mirror 110 is indicated by arrow 130). On the return, the radiation 120b is split by the beamsplitter 115 such that 50% of the radiation is transmitted through the beamsplitter back to the input, and the remainder is reflected to the focal plane array 125. The two beams are recombined at the focal plane array 125. When the position of the movable mirror 110 is varied along the axis of the corresponding arm (indicated by arrow 130), an interference pattern, or interferogram, is swept out at the focal plane array 125 as the two phase-shifted beams interfere with each other.
FIG. 1B illustrates an alternative configuration of an FTS. In this configuration, two focal plane arrays 125a, 125b are used, and the fixed mirror 105 and moving mirror 110 are oriented such that approximately 50% of the radiation 120a, 120b from each optical path is directed to each focal plane array. The spectra from each focal plane array 125a, 125b may be averaged to improve the overall signal-to-noise ratio. This configuration avoids the 50% radiation loss associated with the configuration of FIG. 1A, but is more complex and requires additional components.
As discussed above, in the example FTS configuration of FIG. 1A, the focal plane array 125 receives a modulated interferogram; the modulation being caused by the motion of the movable mirror 110. The focal plane array 125 converts the modulated interferogram to spectral information by measuring the amplitude of cosines at specific frequencies. In the configuration of FIG. 1B, each focal plane array 125a, 125b receives an identical modulated interferogram. The focal plane array(s) converts the modulated interferogram to spectral information by measuring the amplitude of cosines at specific frequencies. FIG. 2 is a graph illustrating an example of the output from the focal plane array 125. The amplitude of each measured frequency (f) is proportional to the amplitude of an incident wavenumber of radiation, ν, according to f=2νV, where V is the velocity of the movable mirror 110. This formula, hereinafter referred to as “Equation (1),” describes that for each incident wavenumber, there is a corresponding interferogram frequency for a given mirror velocity. The graph illustrated in FIG. 2 corresponds to the following example: an input waveband of approximately 8.3-14.3 micrometers (μm) (corresponding to an input wavenumber range of 1200-700 cm−1); a mirror velocity, V, of 0.05 centimeters per second (cm/s), a two second scan, a scan resolution of approximately 10 cm−1, and a sample rate of greater than 120 Hertz (Hz). For this example, Equation (1), f=2νV, yields a measured frequency range of approximately 35-60 Hz.
Polarimetry, or measurement of polarized electromagnetic radiation, may also provide useful information about an object, and typically provides at least some different information than is obtained by spectral imaging. In particular, polarimetry is sensitive to the object orientation, composition, and surface roughness, whereas, spectral information is primarily related to material composition. Therefore, in certain applications, it may be desirable to perform both spectral imaging and polarimetry.
Referring to FIG. 3, an FTS can be converted into a combined spectral imager and polarimeter (spectropolarimeter) by inserting a polarizer 310 into the optical path of the incident electromagnetic beam. Thus, polarized electromagnetic radiation 320 is provided to the FTS and analyzed as described above. The polarizer 310 may be switchable, such that the polarization of the incident electromagnetic radiation may be changed (e.g., from vertical or horizontal, or vice versa). With this arrangement, different polarizations are input, one at a time, to the FTS. Thus, the FTS measures one interferogram at a time (e.g., for either vertical or horizontal polarization). For the configuration illustrated in FIG. 3, the focal plane array 125 receives only ⅛th of the original, unpolarized input radiation 120 because there is a 50% light loss due to transmission through the beamsplitter 115, as discussed above, and the focal plane array 125 measures one polarization (with half the available signal) for half the total time (assuming both polarization measurements will be made). Thus, this arrangement is very inefficient in terms of photon collection efficiency and is susceptible to errors if the object or scene being measured undergoes changes while the inserted polarizer is switched. If the polarizer is not switched then the instrument only measures information in one polarization.